Solid-state electron source



Oct. 26, 1965 LE O APKER 3,214,629

SOLID-STATE ELECTRON SOURCE Filed Aug. 5, 1963 Fig.

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His A fforney' United States Pate 3,214,629 Patented Oct. 26, 1965 lice 3,214,629 SOLID-STATE ELEQTRGN SGURCE Le Roy Aplrer, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Aug. 5, 1963, Ser. No. 299,995 6 Claims. (6!. 313-346) This invention relates to electron emitters and more particularly to a solid-state electron source employing the tunnel effect phenomenon.

It is frequently desirable to provide a source of electrons in a vacuum. Probably the most common device requiring a source of electrons in an evacuated enclosure is the conventional vacuum tube. In such a device the free electrons are normally boiled from an electron-emissive material by direct or indirect heating thereof. The energy required to maintain a suitable operating temperature for electron emission represents a substantial energy loss.

An electron source which can provide electron emission in an evacuated area and yet not require a high operating temperature, with its attendant high power loss, would be Well received in many areas of scientific endeavor. Such an electron source could be used to advantage not only in vacuum tubes but also in other evacuated devices requiring a source of electrons such as mass spectrographs, ion gauges, cathode ray tubes, magnetrons, klystrons, and the like.

Accordingly, it is a principal object of this invention to provide an improved electron source capable of emitting electrons into an evacuated space without requiring elevated temperatures.

It is another object of this invention to provide an efiicient solid-state electron emitter.

In accordance with the present invention a solid-state electron source comprises two layers of semiconductive material separated from each other by an insulating material having a thickness in the order of, or less than, the mean-free path of an electron in the insulating material. In the preferred embodiment of my invention one semiconductor layer consists of N-type silicon and the insulating material conveniently takes the form of silicon dioxide. The other semiconductive layer consists essentially of a material, such as cesium antimonide (Cs Sb), having an electron afi'inity (the energy difference between the bottom of the conduction band at the surface and the vacuum level) lower than the forbidden band gap. When a suitable difference of potential is maintained between the semiconductive layers of the device of my invetnion, electrons tunnel from the N-type semiconductive material through the thin insulating material to the Cs Sb layer, for example, where they emerge from the outer surface thereof into the evacuated region.

The novel features that are characteristic of this invention are set forth with particularity in the appended claims. The invention itself both as to its organization and method of operation, as well as additional objects and advantages thereof will best be understood from the following description when read in conjunction with the accompanying drawing wherein:

FIGURE 1 is a representation of an emitter constructed in accordance with this invention in an evacuated enclosure;

FIGURE 2 is an energy level diagram representing the physical conditions existing during operation of the preferred embodiment of this invent-ion; and,

FIGURE 3 is a representation of an alternative embodiment of the emitter of FIGURE 1.

The phenomenon known as Tunnel elfect is wellknown. One device exhibiting this effect is the tunnel diode which consists of degenerate P-type semiconductive material joined to degenerate N-type semiconductive material by a narrow P-N junction, of the order of angstrom units thick. The junction is made small in order that electrons from the N-type material be enabled to tunnel, under the influence of an electric field, through the forbidden region in the junction. Another example of a device employing the Tunnel effect is the one disclosed in US. Patent No. 3,056,073-Mead. The latter device, as disclosed, consists of two metallic layers separated by an insulating material having a thickness of the order of, or less than, the mean-free path of an electron in the insulating material.

In the device disclosed by Mead, electrons from the top of the Fermi band in the first metal layer tunnel through the insulator to enter the second metal layer. By making the second metal layer thin some electrons will exit therefrom into an evacuated region. However, the fraction of tunneling electrons which enter the second metal layer with sufficient energy to exit therefrom is small. Electron-electron scattering within the second metal layer further diminishes the fraction of electrons reaching the vacuum interface with suflicient energy to permit escape over the barrier thereat.

The result of both degrading phenomena is that the metal-insulator-metal tunnel effect device is reduced in efiiciency as a source of electrons for vacuum devices. Somewhat improved performance attends the use of a dipole layer consisting of a monolayer of adsorbed cesium or oriented barium oxide on the outer surface of the second metal layer because a reduced barrier height is achieved at the vacuum interface.

In accordance with the present invention greatly improved results are achieved by utilizing a. first layer of semiconductive material, preferably having a high impurity concentration, and an insulating layer thereon which may, conveniently, be the oxide of the semiconductive material. I prefer to use a first layer of semiconductive material which is N-type to preferentially supply high energy electrons. The second layer of conductive material is advantageously selected from the group of semiconductive materials having a low electron afiinity and a large migration length for high energy electrons. The first lyer of N-type semiconductive material, with a high impurity concentration, serves as an ample reservoir of high energy electrons to maximize the number of high energy electrons tunneling through the insulating layer. The outer semiconductive material, by virtue of its low electron afiinity maximizes the probability for electrons to escape at the vacuum interface The large diffusion length mitigates against electron-electron scattering and it arises because electrons having energies in the conduction band of magnitude less than the band gap cannot scatter against valence band electrons. Thus, a maximum of electrons reach the vacuum interface with energies that permit them to escape over the barrier at the interface, resulting in efficient emission into the vacuum.

Among the group of semiconductive materials particularly well suited for use as the second semiconductive layer in my invention, are the following: Cs Sb, Rb Sb, Rb Te, K Sb, (Cs)Na KSb, and other semiconductors with electron aflinities appreciably less than the band gap. The band gap of the semiconductor selected lies, preferably, between 2 and 4 electron volts. Higher band gaps characterize insulators which are less desirable. While each of the aforementioned semiconductive materials may be used to advantage singly or in combination with other semiconductive materials in the practice of my invention, I prefer to use CSgSb, because I have discovered that this material provides a particularly efiicient emitter of electrons. This material has a low electron affinity and large migration lengths for high energy electrons in the conduction band with energies less than the band gap, of about two electron volts. Electrons which tunnel through from the first layer having energies greater than two electron volts are quickly degraded in the conduction band by pair-production after which they do not degrade further except by relatively weak processes such as lattice scattering. A large percentage of the tunneling electrons reach the vacuum-Cs Sb interface. Additionally, the elec trons are helped over the barrier at this vacuum interface because Cs Sb, which is normally P-type, becomes less P-type at the interface. The resultant electric field aids the escape of electrons into the vacuum by accelerating the electrons toward the vacuum.

By measuring the yield for photoelectric emission, I have observed that more than 20 percent of the electrons reaching the Cs Sb-vacuum interface exit into the vacuum. Thus, a significant advantage over the prior art structures is achieved not only with respect to the number of electrons reaching the interface, but also the percentage escaping therefrom into the vacuum is increased relative to metal second layers (such as gold) wherein the percentage of electrons surmounting the vacuum interface barrier is normally in the order of ten percent or lower.

FIGURE 1 illustrates, schematically, a structure embodying my invention. There is shown therein a partial cross-sectional view of an evacuated region 1 defined by an hermetically sealed envelope 2 which includes an emitter-supporting portion 3. Envelope 2 may, for example, be constituted of glass or quartz in a manner Well-known in the vacuum tube art. In addition, envelope 2 may include various current controlling and collecting electrodes (not shown). Envelope 2 may enclose the grid and anode elements of a conventional vacuum tube in which case emitter-supporting portion 3 would be variously described in that art as a base, stem or header.

In accordance with the present invention there is provided in evacuated region 1 an electron emitter 4 comprising a first layer 5 of semiconductive material and a second layer 6 of semiconductive material separated by an insulating layer 7 of thickness equal to, or less than, the mean free-path of electrons therein.

In FIGURE 1 means to provide electric circuit connections to semiconductive layers 5 and 6 take the form of electrical conductors 8 and 9 which extend through envelope 2 into evacuated region 1 where they are connected to layers 5 and 6, respectively. In order to preserve the vacuum in evacuated region 1, conductors 8 and 9 are hermetically sealed at the point of passage through envelope 2 by any of a plurality of means well-known in the art. Emitter 4 is held in place by conductors 8 and 9 or, alternatively, may be secured to portion 3 by an inert adhesive.

Conductors 8 and 9 are connected outside envelope 2 to terminals 10 and 11 respectively which are arranged to be connected to a suitable source of power having a voltage magnitude sufficient to effect tunneling of electrons from semiconductive layer 5 through insulating layers 7 to semiconductive layer 6. Emission of electrons from layer 6 into evacuated region I can be controlled by modulating the source of voltage connected to terminals 10 and 11.

In accordance with the preferred embodiment of my invention emitter 4 is constituted of a layer 5 of N-type degenerate silicon, an insulating layer '7 of silicon dioxide and an upper layer 6 of Cs Sb semiconductive material. Such a structure may conveniently be produced by providing a body of high purity silicon and introducing N- type impurity therein by diffusion of antimony, arsenic, or phosphorous. Alternatively, the silicon crystal can be grown from a melt rich in donor impurity. Thereafter, the silicon body is heated in any oxygen atmosphere to a temperature of about 140 C. for from about 5 to hours to provide a thin layer of silicon oxide having a thickness in the order of about 100 Angstrom units or less. This thickness is less than the mean free-path of an electron in silicon oxide insulating material, which is in excess of Angstrom units. In this way, N-type degenerate silicon semiconductive material layer 5 and silicon dioxide insulating layer 7 are formed.

Thereafter, Cs Sb semiconductive material is deposited on the outer surface of insulating layer 7 by any of a plurality of means well-known in the art. I prefer to deposit layer 6 by first evaporating Sb, and then introducing an atmosphere of Cs at a temperature of from 100 to for from 10 minutes to 1 hour. Electrical connections are made to layers 5 and 6 by evaporated metals or by any other means well-known to the art.

FIGURE 2 illustrates an energy level diagram of the SiSiO -Cs Sb emitter of FIGURE 1. Under the influence of an electric field, which may be provided conveniently by a voltage source which maintains terminal 11 about 1 to 10 volts positive With respect to terminal 10, electrons leave the densely electron-populated region 12 of the conduction band of Si, tunnel through the thin insulating layer of SiO pass through the Cs Sb semiconductive layer and those electrons having energies such as electron 13 surmount the vacuum interface barrier and provide electron emission into the vacuum.

FIGURE 3 represents an alternative embodiment of the invention wherein a conducting grid 14 is applied over semiconductor 6 of the electron emitter shown in FIG- URE 1. Since the grid is in contact with semiconductor 6, its presence results in improved lateral conductivity of semiconductor 6. The grid may be effected in any of a plurality of ways known in the art, including evaporation of a conductor over a mask.

While I have shown and described my invention with respect to several specific embodiments thereof, many modifications and variations of my invention will suggest themselves to those skilled in the art. For example, the solid-state electron emitter of my invention is not constrained to a generally planar emitting surface but may equally well take the form of a sphere, a parabola or other geometrical configurations to particularly suit a specific application. Also, it is not required that the emitter be totally enclosed Within an evacuated area since the emitter may, itself, form a large or small portion of the envelope which encloses the evacuated region by merely providing a suitable coating of inert material on the outside of the semiconductive layer which acts as a source of electrons. Therefore, it is intended by the appended claims to include these and other modifications and variations which fall within the true spirit and scope of my invention.

What I claim as new and desire by Letters Patent of the United States is:

1. A solid-state electron source comprisin: an emissive layer consisting essentially of Cs Sb semiconductive material; a second layer consisting essentially of N-type semiconductive material, said second layer being separated from said emissive layer by an insulating material having a tluckness in the order of, or less than, the mean-free path of an electron in said insulating material; and means for providing electric circuit connections to said emissive and second layers.

2. The source of claim 1 having a conductive grid connected to and disposed on the outer surface of said emissive layer.

3. A solid-state electron source comprising: a first layer consisting of P-type semiconductive material having a forbidden band gap of from 2 to 4 electron volts and an electron afiinity less than said band gap; a second layer consisting of N-type semiconductive material, said secand layer being separated from said first layer by an insulating material having a thickness in the order of, or less than, the mean-free path of an electron in said insulating material; and means for providing electric circuit connections to said first and second layers.

4. The source of claim 3 having a conductive metal grid connected to the outer surface of said first layer and increasing the laterial conductivity thereof.

5. A solid-state electron source comprising: an emissive layer consisting essentially of at least one P-type compound selected from the group consisting of Cs Sb, Rb Sb, Rb Te, K Sb, and (Cs) Na KSb semiconductive material; a second layer consisting essentially of N-type semiconductive material, said second layer being separated from said emissive layer by an insulating material having a thickness in the order of, or less than, the mean-free path of an electron in said insulating material; andmeans for providing electric circuit connections to said emissive and second layers.

6. A solid-state electron source comprising: an emissive layer consisting essentially of Cs Sb semiconductive material; a second layer consisting essentially of degenerate N- type silicon semiconductive material, said second layer being separated from said emissive layer by an insulating material of SiO having a thickness in the order of, or less than, 150 Angstrom units; a grid of highly conductive material contiguous with said emissive layer and increasing the lateral conductivity thereof; and, means for providing electric circuit connections to said emissive and second layers.

References Cited by the Examiner UNITED STATES PATENTS 3,024,140 3/62 Schmidlin 317-235 3,056,073 9/62 Mead 317-238 X 3,098,168 7/63 Aigrain 313-346 3,116,427 12/63 Giaever 317-235 3,150,282 9/64 Geppert 313-346 JOHN W. HUCKERT, Primary Examiner. JAMES D. KALLAM, Examiner. 

1. A SOLID-STATE ELECTRON SOURCE COMPRISIN: AN EMISSIVE LAYER CONSISTING ESSENTIALLY OF CS3SB SEMICONDUCTIVE MATERIAL; A SECOND LAYER CONSISTING ESSENTIALLY OF N-TYPE SEMICONDUCTIVE MATERIAL, SAID SECOND LAYER BEING SEPARATED FROM SAID EMISSIVE LAYER BY AN INSULATING MATERIAL HAVING A THICKNESS IN THE ORDER OF AN INSULATING MATERIAL HAVING PATH OF AN ELECTRON IN SAID INSULATING MATERIAL; AND MEANS FOR PROVIDING ELECTRIC CIRCUIT CONNECTIONS TO SAID EMISSIVE AND SECOND LAYERS. 